diff --git a/bin/sassw b/bin/sassw
index 0aa0b35..a8f0c3e 100755
--- a/bin/sassw
+++ b/bin/sassw
@@ -26,6 +26,7 @@ else:
 def build():
     global sources, directories
     directories = set()
+    sources = set()
     for filename in filenames:
         result = subprocess.run(
                 ['sassc', '-mauto', filename, filename.replace('.scss', '.css')],
@@ -43,7 +44,8 @@ def build():
                          }''' % result.stderr.replace('\n', '\\A').replace('"', '\\"'),
                     file=fd)
         basepath = os.path.abspath(os.path.dirname(filename))
-        sources = [os.path.abspath(os.path.join(basepath, x)) for x in json.load(open(filename.replace('.scss', '.css.map')))['sources']]
+        sources = sources.union(set([os.path.abspath(os.path.join(basepath, x))
+            for x in json.load(open(filename.replace('.scss', '.css.map')))['sources']]))
         directories = directories.union(set([os.path.dirname(x) for x in sources]))
 
 class EventManager(pyinotify.ProcessEvent):